<input type="text" value="Москв" />
<input type="radio" name="один" />
<input type="radio" name="два" />
<script>
var input = document.querySelector('input[type="text"]');
var radio = document.querySelector('input[type="radio"][name="один"]');
var updateRadioState = function() {
radio.disabled = input.value == "Москва";
};
input['oninput' in input ? 'oninput' : 'onpropertychange'] = updateRadioState;
</script>